The present invention relates to an optical attenuator and more specifically to an optical attenuator used for attenuating optical signals in the fields of optical communications, optical measurements, CATV systems and the like.
Optical attenuators comprising an optical fiber containing an optical attenuating dopant have been widely known. However, the dopant contained in these generally known optical attenuators has a transmitted light attenuating characteristic where the attenuation varies depending on the wavelength of the optical signal, i.e., it has a wavelength dependency. There is also known an optical attenuator in which the wavelength dependency is reduced by adjusting the mode field diameter of the optical fiber and by limiting the dopant area with respect to the mode field diameter in order to obtain almost equal attenuation of input optical signals of different wavelengths, e.g., 1.3 xcexcm (short wavelength) and 1.5 xcexcm (long wavelength) (Japanese Laid-Open (xe2x80x9cKokaixe2x80x9d) Nos. Hei. 8-136736 and Hei. 8-136737).
Recent diversification of optical communications, has created a demand for an optical attenuator having equal optical attenuation (eliminating the wavelength dependency) even in a narrow wavelength range of 1300 mmxc2x150 nm or 1550xc2x150 nm, for example or an optical attenuator whose wavelength dependency in optical attenuation is increased for optical signals of, for example, two different wavelengths of 1.3 xcexcm (short wavelength) and 1.5 xcexcm (long wavelength).
However, although the optical attenuators disclosed in Japanese Laid-Open (xe2x80x9cKokaixe2x80x9d) Nos. Hei. 8-136736 and Hei. 8-136737 are effective because they give almost equal attenuation of optical signals of two different wavelengths of 1.3 xcexcm (short wavelength) and 1.5 xcexcm (long wavelength), they have the problem that they are unable to provide equal optical attenuation (wavelength dependency is large) merely by limiting the dopant area or by adjusting the mode field diameter when the difference of the wavelengths is small.
When the optical signals of two different wavelengths of (short and long) wavelengths are input, it is theoretically possible to increase the wavelength dependency of the optical attenuation by using a dopant which gives greater attenuation of the short wavelength optical signals, with high concentration close to the axial core when the mode field is seen as a transverse section of the optical fiber or by using a dopant which gives greater attenuation of longwave optical signals with higher concentration close to the outer periphery of the optical fiber when the mode field is seen as a transverse section of the optical fiber (Japanese Laid-Open (xe2x80x9cKokaixe2x80x9d) No. Hei. 8-136736). 
It is also theoretically possible to realize the equality (Japanese Laid-Open (xe2x80x9cKokaixe2x80x9d) No. Hei. 8-136737) by reversing the combination of the wavelength characteristics of the mode field diameter and the wavelength characteristics of the dopant.
However, although the difference between the short wavelength and the long wavelength attenuation is increased by raising the dopant concentration and by limiting the doping area to a narrow range with respect to the mode field diameter, there has been a difficult problem that, because the doping concentration of the dopant which can be contained in the optical fiber is limited, it is not possible to create an optical fiber product having characteristics which are stable when the concentration is too high and it is not technologically possible to create optical fiber products whose doping area is very narrow.
The present invention has been devised in view of the above-described problems and has as its object, firstly, to provide an optical attenuator which can equalize optical attenuation of optical signals having different wavelengths which are very close and, secondly, to provide an optical attenuator which can maximize the difference of optical attenuation of the optical signals having different wavelengths in an optical fiber with stable characteristics and wherein the dopant concentration and doping area range may be realized with a relatively low dopant concentration.
In order to achieve the above-mentioned objects, the present invention provides a single mode optical fiber, as an inventive optical attenuator, having a core with a refractive index of a center portion greater than that of a peripheral portion.
The wavelength dependency of the attenuation of transmitted light caused by the size of the mode field diameter is increased by adopting, as a distribution of the refractive index of the core, a distribution gradient selected from the group consisting of a gradient wherein the refractive index rises continuously from the peripheral portion to the center portion (xe2x80x9cgraded-index typexe2x80x9d), a parabolic shaped gradient, a triangular wave shaped gradient, a square wave shaped gradient and a trapezoidal wave shaped gradient.
By constructing the optical attenuator as described above, it is possible to widen the limited width of the dopant area for obtaining the required attenuating characteristics as much as possible and to minimize the dopant concentration.
In one embodiment the optical attenuator contains dopant which provides greater attenuation of longer wave length transmitted light in a signal mode optical fiber and is constructed so that the dopant area is limited to the center part of the core and so that the refractive index at the center part of the core is greater than that of the peripheral part of the core. The wavelength dependency of the attenuation of transmitted light caused by the size of the mode field diameter is increased by adopting, as the distribution of refractive index of the dopant area, a gradient selected from the group consisting of a gradient wherein the refractive index rises continuously from the peripheral portion to the center portion (xe2x80x9cgraded-index typexe2x80x9d), a parabolic shaped gradient, a triangular wave shaped gradient, a square wave shaped gradient and a trapezoidal wave shaped gradient. By constructing the optical attenuator in this manner, it is possible to obtain equal attenuation of two input optical signals having different wavelengths which are short and whose difference is small (1300 nmxc2x150 nm).
In another embodiment the optical attenuator is a signal mode optical fiber containing dopant which provides greater attenuation of shorter wavelength transmitted light and constructed so that the dopant area is limited to the peripheral part of the core and so that the refractive index at the center part of the core containing no dopant is greater than that of the peripheral part of the core. In this embodiment also, the wavelength dependency of the attenuation of transmitted light caused by the size of the mode field diameter is increased by adopting a refractive index gradient selected from the group consisting of a gradient wherein the refractive index rises continuously from the peripheral portion to the center portion, a parabolic shaped gradient, a triangular wave shaped gradient, a square wave shaped gradient and a trapezoidal wave shaped gradient, as the refractive index profile at the center part of the core where no dopant is contained. By constructing the optical attenuator in this manner, it is possible to obtain equal attenuation of two kinds of input optical signals having different long wavelengths whose difference is small (1550 nmxc2x150 nm). In still another embodiment an optical attenuator is a signal mode optical fiber containing a dopant which preferentially attenuates shorter wavelength transmitted light and is constructed so that the dopant area is limited to the center part of the core and so that the refractive index at the center part of the core containing dopant is greater than that of the peripheral part of the core. In this case, the wavelength dependency of the attenuation of transmitted light caused by the size of the mode field diameter is increased by adopting a refractive index profile selected from the group consisting of a gradient wherein the refractive index rises continuously from the peripheral portion to the center portion, a parabolic shaped gradient, a triangular wave shaped gradient, a square wave shaped gradient and a trapezoidal wave shaped gradient. By constructing the optical attenuator in the above manner, it is possible to obtain optical signals of two different wavelengths whose difference in attenuation of transmitted light caused by the difference of the wavelengths is maximized.
Another embodiment provides an optical attenuator in the form of a signal mode optical fiber containing dopant which gives greater attenuation of longer wavelength transmitted light and constructed so that the dopant area is limited to the peripheral part of the core. A gradient wherein the refractive index rises continuously from the peripheral portion to the center portion (xe2x80x9cgraded-index typexe2x80x9d) is adopted as the refractive index profile of the dopant area to increase the wavelength dependency of attenuation of transmitted light caused by the size of the mode field diameter.
In yet another embodiment an optical attenuator is constructed as a single mode optical fiber having a refractive index at the center part of the core greater than that of the peripheral part of the core due to incorporation of a dopant whose transmitted light attenuating characteristics depend on the wavelength of optical signal input to the optical fiber. The dopant concentration of the dopant area of the single mode optical fiber is distributed non-uniformly to provide a mode field which substantially contributes to the transmission of optical signals in the radial direction, i.e., transverse of the optical fiber. In this case, the wavelength dependency of the attenuation of transmitted light caused by the size of the mode field diameter is increased by adopting, as the distribution of refractive index of the dopant area, a gradient selected from the group consisting of a gradient wherein the refractive index rises continuously from the peripheral portion to the center portion, a parabolic shaped gradient, a triangular wave shaped gradient, a square wave shaped gradient and a trapezoidal wave shaped gradient. By constructing the optical attenuator in this manner, it is possible to obtain the required attenuating characteristics even when the dopant area is small and the dopant concentration is low.